When certain binary memory cells are struck by ionizing radiation, they may exhibit "soft" errors which change the state of the cell without affecting its capacity to later reliably receive and store binary information. However, such "soft" errors corrupt the data stored in memory. To avoid erroneous data, circuitry external to the memory circuitry is commonly provided, which detects and corrects errors as by detecting which one of the several bits of a byte or word is in error and by substituting the correct bit for the erroneous one. Of course, additional bits are required in each word in order to correct an error. The ratio of information bits in each word to the total number of bits in the word represents the "information rate" of the system.
One example of a memory subject to soft errors is an interplanetary space probe which must sometimes make autonomous decisions based on programs stored in its memory. The memory in such space probes is subject to soft errors caused by cosmic radiation. In order to reduce the amount of memory and power consumption of the spacecraft, it is desirable to provide a maximum information rate, that is, to provide a memory system wherein a maximum percentage of the memory is devoted to information and a minimum to redundancy for error correction, while still providing a high degree of error correcting capability.
There are two classes of semiconductor RAMs (random-access-memories), these being static (S) and dynamic (D) types. DRAMs are easily upset by cosmic rays, and are therefore unsuitable for space flight. SRAMs are more suitable. Commercial SRAMs are designed with balanced memory cells. That is, the memory cells are equally likely to change from a first state to a second state, as from a second state to a first state, when subjected to radiation that can cause soft errors. If the SRAM cells could be unbalanced, then the error rate would be greater for a first to a second state than for a second to a first state. If such asymmetry were used to increase the information rate (ratio of information bits to total bits in an error correcting memory system), it would provide more efficient memory systems.